Bump black from 19.10b0 to 20.8b1 (#528)

Author: dependabot[bot]

requirements/requirements_dev.txt

@@ -1,7 +1,7 @@
 # https://github.com/HypothesisWorks/hypothesis/issues/2113 
 attrs>=19.2.0
 bandit==1.6.2
-black==19.10b0
+black==20.8b1
 dlint==0.10.3
 flake8==3.8.3
 flake8-bandit==2.1.2